1. Field of the Invention
The present invention generally relates to means for testing the constituent memory elements of computer memory arrays and, more particularly, to apparatus for quickly testing arrays comprising memory elements of the recirculating type, such as, for example, charge coupled devices.
2. Description of the Prior Art
In charge coupled device (CCD) memory arrays, multiple CCD loops are organized so as to be accessible via a single input pin and via a single output pin. Accordingly, the loops are accessed in serial succession. In those cases where the entire array is to be initialized to a predetermined state or tested to verify that the stored data is correct, sufficient time must be allocated to complete the serial accessing of the CCD loops. The required time increases objectionably as the CCD memory array increases in density to include larger numbers of individual loops. Direct parallel accessing of the individual CCD loops, on the other hand, permits much more rapid initialization, but only at the expense of requiring unacceptably large numbers of input-output (I/O) pins.
Indirect parallel accessing of interior logic circuit points has been achieved via single I/O pins according to a technique described in U.S. Pat. No. 3,783,254 to E. B. Eichelberger for "Level Sensitive Logic System", issued Jan. 1, 1974 and assigned to the present assignee and in related U.S. Pat. Nos. 3,761,695 and 3,784,907 to the same inventor. Briefly, clocked dc latches are provided at logic network nodes to be tested and additional circuitry is included to selectively connect the latches into a functional shift register. A predetermined pattern of binary ones and zeros can then be introduced serially into the shift register latches where they are retained for later use as parallel inputs to the logic network nodes to be dc tested. This technique, however, is not well suited for the initializing and dynamic testing of CCD loops within a memory array.